Method of bleaching cellulose fibres



3,318,657 METHOD OF BLEACHING CELLULOSE FIBRES Robert C. Wade, Ipswich, Mass, assignor to Metal Hydrides Incorporated, Beverly, Mass., a corporation of Massachusetts No Drawing. Filed Sept. 23, 1963, Ser. No. 310,862 4 Claims. (Cl. 8-111) This invention relates to bleaching cellulose fibres, such as jute, linen, cotton, rayon and wood.

It has long been known that ozone is a powerful oxidizing agent capable of oxidizing many organic materials. In previous attempts to use ozone to oxidize the colored impurities in cellulosic materials, such as jute, linen, cotton, rayon and wood, not only were the impurities oxidized and rendered colorless but the cellulosic material itself was attacked and oxidized so severely that it was rendered useless for its subsequent intended use.

It is obvious that if the degradative nature of ozone on cellulosic fibres could be overcome, great economies of bleaching these materials might be realized. This stems from the fact that ozone can be generated industrially at a very low cost. As an oxidizing agent it is far cheaper than hydrogen peroxide, or chlorine dioxide now widely used in bleaching cellulosic fibres. In fact, its cost is about the same as sodium hypochlorite, which is also used widely but has other limitations.

The present invention is based upon the discovery that the above mentioned detrimental action of ozone on cellulosic fibres can be substantially overcome and good bleaching of the fibres obtained if the fibres are subjected to the action of an aqueous solution of an alkali metal borohydride, such as sodium or potassium borohydride, before subjecting the fibres to the action of ozone.

Because of the wide variety of cellulosic fibres amenable to bleaching by this technique and because of the difference in bleachable impurities in the different fibres, and in the many varieties of fibres of a single type and their method of manufacture (e.g., the large variety of wood pulps), the optimum conditions for practicing this invention will vary rather widely. *For instance, wood pulps can be treated under far more drastic conditions than can rayon fibres. In fact, because of the large amount of bleachable impurities in Wood compared to rayon or cotton, more drastic bleaching conditions are generally desirable. Regardless of the fibre source, however, the combination of treating the fibre first with an alkali metal borohydride followed by treatment with ozone gives a superior bleached and brightened fibre.

In the preferred practice of the invention the amount of borohydride used should be between about 0.01 and 2 percent based upon the dry weight of the fibre. While greater amounts of borohydride can be used, it is unnecessary and becomes uneconomical. The concentration of borohydride in the water is not critical but is entirely dependent upon the method of application to the fibre. Thus, when treating wood fibres very large volumes of water relative to the wood are used and the borohydride concentration can be very low. In treating woven textile fibres, the solution can be padded on the fabric and since the normal weight pick-up of solution is about 100%, the concentration of borohydride might be 0.1 to 0.5%. When the fibres are sprayed with relatively small amounts of solution based upon the weight of the fibre, more concentrated solutions of borohydride can be used.

The pH of the borohydride solution should be above 9 and in many cases it should be applied with at least 1% of sodium hydroxide present in the solution. Wood fibres in particular should be treated with borohydride solutions stablized with excess caustic soda, caustic potash, or lime. These bases prevent excessive hydrolytic that utilisation... Mauritan an 3,318,657 Patented May 9, 1967 decomposition of the borohydride while it is in contact with the fibre.

The time and temperature for fibre treatment with borohydride can vary widely. In general this may be from 4 hours at 20 C., l to 2 hours at 30 to 35 C., to 2 to 10 minutes at to C. Higher temperatures can be employed by operating this treatment under pressure, thereby reducing the contact time necessary.

After treatment with the alkali metal borohydride the fibres are prepared for treatment with ozone by lowering their pH to at least 7 and, preferably, to a pH of about 4. This may be accomplished by treating the fibres with a solution of an acid, such as hydrochloric, sulfurous, sulfuric, acetic acid, etc., or by washing the fibres to remove the alkaline materials prior to treatment with the acid solution.

The ozone treatment can be carried out in an aqueous solution having a pH between 2 and 7 and preferably between 2 and 4, or by exposing the wet fibres to an atmosphere containing ozone in a concentration of up to 6% by weight, the preferred concentration being about 2 percent. The time of exposure to give the desired bleaching must be determined for each fibre and is related to ozone concentration and temperature. should be adjusted so that about 0.1 to 3% of ozone based on the weight of the dry fibre is consumed during the process. Exposure time may require 2 hours at room temperature to 2 minutes or less at 100 C. Again the exact conditions for each fibre must be tested individually. After ozone treatment the fibre is again washed and if desired adjusted to a pH required for further processing or finishing.

In the above described technique bleached cellulosic fibres are obtained which are generally 5-10% stronger or less degraded than fibres which have been treated with ozone but have omitted the borohydride pretreatment.

The invention is illustrated further by the following specific example. A sample of cotton cloth one and onehalf inches wide and thirteen inches long, weighing 2.2 grams, was placed in an aqueous solution containing 0.2 percent of sodium borohydride by weight, the sample remaining in the solution for one-half hour. The sample was then wrung out to approximately 100% wet pick-up and placed in a resin kettle on a wire hanger and ozone was passed through at a rate of 1.4 liters per minute for one-half hour, the ozone concentration being 14 mg. per liter. The amount of sodium borohydride picked up by the cloth was 4.4 mg. The treated cloth had a brightness of 70.5 points and a tensile strength of 55.6 pounds per square inch, the original brightness being 64 points and the original tensile strength being 54.6 pounds.

In the same experiment described above, another piece of cotton which was not pretreated with sodium borohydride but only wetted with water to 100% weight pick-up was placed in the resin kettle and exposed to ozone as described above. The average brightness of this sample was only 65.5 points and the tensile strength was only 53.2 pounds.

I claim:

1. The method for bleaching cellulose fibres which comprises subjecting the fibres to treatment with an aqueous solution of an alkali metal borohydride having a pH of at least 9, and thereafter treating the fibres with ozone in an aqueous medium having a pH between about 2 to 7.

2. The method as described by claim 1 wherein said treatment with ozone is conducted at a temperature between room temperature and about 100 C. and is such that about 0.1 to about 3 percent by weight of ozone based upon the weight of the dry fibres is consumed and wherein said treatment with a solution of an alkali metal Thesev borohydride is conducted at a temperature between room References Cited by the Examiner temperature and about 100 C., said solution containing UNITED STATES PATENTS from about 0.01 to about 2 percent by weight of an alkali metal borohydride based upon the weight of the dry fibres. 1,760,042 5/1930 Ores!1 et 3. The method as described by claim 1 wherein said 5 3,017,316 1/1962 Rapson 8-4085 alkali metal borohydride is selected from the group con- 3,081,265 3/1963 n 8 108 sisting of sodium borohydride and potassium borohydride. 3,198,651 8/1965 Shlralshl 8 111 X 4. The method as described by claim 2 wherein said NORMAN G. TORCHIN, Primary Examiner. alkali metal borohydride is selected from the group J TRAVIS BROWN Examiner consisting of sodium borohydride and potassium boro- 10 hydride. I T. J. HERBERT, Assistant Examiner. 

1. THE METHOD FOR BLEACHING CELLULOSE FIBRES WHICH COMPRISES SUBJECTING THE FIBRES TO TREATMENT WITH AN AQUEOUS SOLUTION OF AN ALKALI METAL BOROHYDRIDE HAVING A PH OF AT LEAST 9, AND THEREAFTER TREATING THE FIBRES WITH OZONE IN AN AQUEOUS MEDIUM HAVING A PH BETWEEN ABOUT 2 TO
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